Currently available microelectromechanical systems (“MEMS”) typically are produced from very thin films (e.g., on the order of four micrometers thick). Such thin films have the undesirable quality of being relatively fragile, even when made from relatively strong materials, such as silicon. Accordingly, without some means for protecting the internal MEMS structure, MEMS devices would not be very robust for many applications. For example, without some means for protecting its basic structure, the flexures supporting a movable mass of a MEMS accelerometer may crack when subjected to anticipated accelerations.
The art has responded to this problem by filling the interior cavities of MEMS devices with relatively high viscosity gasses. More specifically, high viscosity gasses maintain MEMS structural integrity by effectively cushioning movable components of a MEMS device. A high viscosity gas within an accelerometer therefore should cushion the movable mass and its flexures when subjected to anticipated accelerations. Accordingly, this cushioning effect should maintain the internal MEMS components in operable condition. Exemplary gasses used for these purposes include nitrogen, which has a viscosity of about 17.6 micropascal-seconds at 20 degrees C., and ambient air, which has a viscosity of about 18.2 micropascal-seconds at about 20 degrees C.
In addition to providing a cushioning effect, however, high viscosity gasses in MEMS devices frequently generate noise (referred to in the art as “Brownian Noise”). In fact, such noise often can impede the basic function of the MEMS device.